1. Field Of The Invention
The subject matter of the present invention is a method of preparing 1,1-difluoroethylene in which acetylene is used as the starting product, and in which no intermediate products are isolated.
2. Discussion Of The Prior Art
The preparation of 1,1-difluoroethylene is, as it is known, performed in three steps: in the first step, 1,1-difluoroethane is obtained by the hydrofluorination of acetylene. Thereafter, the purified 1,1-difluoroethane is chlorinated to form 1,1-difluoro-1-chloroethane which is isolated from the chlorination products. In a third step, hydrogen chloride is split off thermally from the 1,1-difluoro-1-chloroethane with the formation of 1,1-difluoroethylene.
It is known to hydrofluorinate acetylene in the presence of a catalyst consisting of a tableted mixture of aluminum fluoride and bismuth fluoride. By means of this catalyst no more than 97% of the acetylene can be reacted. The reaction products contain 70 to 72 wt.-% of 1,1-difluoroethane and 25 to 27 wt.-% of vinyl fluoride. Prior to the further processing of a product contained in such a mixture, the mixture must be refined by distillation before using the 1,1-difluoroethane in further reactions.
As a catalyst for the hydrofluorination of acetylene to 1,1-difluoroethane, a catalyst containing aluminum fluoride has been proposed which is prepared by imbibing .gamma.- or .eta.-aluminum oxide with a bismuth salt and manganese salt solution, drying the aluminum oxide impregnated with the salts at temperatures up to 100.degree. C., and then heating it, at first in a nitrogen atmosphere, and later in air with increasing concentrations of hydrogen fluoride up to a 100% hydrogen fluoride atmosphere, at temperatures between 150.degree. and 250.degree. C. The preparation of such a catalyst has been described, for example, in German Offenlegungsschrift No. 20 00 200. The reaction product obtained by hydrofluorination with such a catalyst can contain up to 2% of acetylene and a maximum of about 4.5% of vinyl fluoride. Consequently, both of the by-products have hitherto always been separated prior to any further processing of the difluoroethane.
The further processing of a purified 1,1-difluoroethane to 1,1-difluoroethylene in a one-step process has already been described in U.S. Ser. No. 800,605, assigned to the assignee hereof, the disclosure of which is hereby incorporated herein by reference. In the method described in this application, 1,1-difluoroethane is photochlorinated to 1,1-difluoro-1-chloroethane and immediately thereafter the reaction products obtained are heated at temperatures between 550.degree. and 750.degree. C. without isolation of the 1,1-difluoro-1-chloroethane.